FIG. 1 is a block diagram illustrating an example of a conventional ring-type broadband communications system, such as a two-way hybrid fiber/coaxial (HFC) network. It will be appreciated that other networks exist, such as a star-type network. These networks may be used in a variety of systems, including, for example, cable television networks, voice delivery networks, and data delivery networks to name but a few. The broadband signals transmitted over the networks include multiple information signals, such as video, voice, audio, and data, each having different frequencies. Headend equipment included in a headend facility 105 receives incoming information signals from a variety of sources, such as off-air signal source, a microwave signal source, a local origination source, and a satellite signal source and/or produces original information signals at the facility 105. The headend 105 processes these signals from the sources and generates forward, or downstream, broadcast signals that are delivered to a plurality of subscriber equipment 110. The broadcast signals can be digital or analog signals and are initially transported via optical fiber 115 using any chosen transport method, such as SONET, gigabit (G) Ethernet, 10 G Ethernet, or other proprietary digital transport methods. The broadcast signals are typically provided in a forward bandwidth, which may range, for example, from 45 MHz to 870 MHz. The information signals may be divided into channels of a specified bandwidth, e.g., 6 MHz, that conveys the information. The information is in the form of carrier signals that transmit the conventional television signals including video, color, and audio components of the channel. Also transmitted in the forward bandwidth may be telephony, or voice, signals and data signals.
Optical transmitters (not shown), which are generally located in the headend facility 105, convert the electrical broadcast signals into optical broadcast signals. In most networks, the first communication medium 115 is a long haul segment that transports the signals typically having a wavelength in the 1550 nanometer (nm) range. The first communication medium 115 carries the broadcast optical signal to hubs 120. The hubs 120 may include routers or switches to facilitate routing the information signals to the correct destination location (e.g., subscriber locations or network paths) using associated header information. The optical signals are subsequently transmitted over a second communication medium 125. In most networks, the second communication medium 125 is an optical fiber that is typically designed for shorter distances, and which transports the optical signals over a second optical wavelength, for example, in the 1310 nm range.
From the hub 120, the signals are transmitted to an optical node 130 including an optical receiver and a reverse optical transmitter (not shown). The optical receiver converts the optical signals to electrical, or radio frequency (RF), signals for transmission through a distribution network. The R-F signals are then transmitted along a third communication medium 135, such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers 140 positioned along the communication medium 135. Taps (not shown) further split the forward RF signals in order to provide the broadcast RF signals to subscriber equipment 110, such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one subscriber location 110 is shown for simplicity, however, each distribution branch may have as few as 500 or as many as 2500 subscriber locations. Additionally, those skilled in the art will appreciate that most networks include several different branches connecting the headend facility 105 with several additional hubs, optical nodes, amplifiers, and subscriber equipment. Moreover, a fiber-to-the-home (FTTH) network 145 may be included in the system. In this case, optical fiber is pulled to the curb or directly to the subscriber location and the optical signals are not transmitted through a conventional RF distribution network.
In a two-way network, the subscriber equipment 110 generates reverse RF signals, which may be generated for a variety of purposes, including video signals, e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream in a typical United States range from 5 MHz to 40 MHz through the reverse path to the headend facility 105. The reverse RF signals from various subscriber locations are combined via the taps and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment 110.
The preferred specification for a communications system, such as a cable television system, is to have a constant RF output level at the amplifier input or at the subscriber equipment input. Disadvantageously, various factors, such as transmitter output drift or connector loss changes, contribute negatively to the RF output level, and consequently degrade the quality of the signal preventing a constant level. Accordingly, communications equipment include correction circuits in order to adjust for these negative factors that cause variation in the RF output level. Thus, the present invention is a circuit directed towards ensuring a constant RF output level in optical receivers that are suitable for use in the communications system of FIG. 1.